This invention relates to the field of remote passive detection of air temperature gradients to provide early warning to aircraft of clear air turbulence and wind shear conditions.
Remote sensing of air temperature gradients is a known means for detecting air turbulence conditions, such as clear air turbulence and wind shear. Both of these conditions can pose serious hazards to aerial navigation. Generally, known procedures involve sensing the infrared radiation in a column of air ahead of the aircraft to determine changes in the spatial temperature profile which are associated with aerial turbulence, as described by R. W. Astheimer in "The Remote Detection of Clear Air Turbulence by Infrared Radiation", Appl. Optics, 9 (1970) 1789. This has involved detection of radiation in the 13 to 14.5 .mu.m CO.sub.2 regions by means of Fabry-Perot interferometers, as described by Astheimer. Others have proposed similar systems utilizing CO.sub.2 radiation in the 13 to 15.5 .mu.m band (U.S. Pat. Nos. 3,735,136, 3,780,293, and 3,935,460, all to Flint); in the 27 to 33 .mu.m water vapor band (U.S. Pat. No. 4,266,130 to Kuhn, and U.S. Pat. No. 4,427,306 to Adamson); or radiation from oxygen molecules (U.S. Pat. No. 3,359,557 and 3,380,055, both to Fow et al.).
It is also known to sense radiation at different freqencies to obtained depths-temperature profiles (see for example U.S. Pat. No. 3,380,055 to Fow et al., and the aforementioned Astheimer article). This is based on the observation that strong spectral bands will have high emission but also high absorption, hence will provide information only at close ranges since radiation originating at longer ranges will be strongly absorbed. Conversely, weak bands with low absorption and low emission allow probing at longer ranges. Therefore, by comparing observations at different wavelengths representing relatively stronger and weaker bands, one can observe changes over time in the temperature profile in the column of air.
Prior art remote temperature sensing methods had used relatively coarse (wide spectral bandwidth) interference filters, which mass many spectral lines, to isolate spectral regions with weak absorption in an attempt to obtain range information. Accurate range information could not be obtained by these instruments because the coarse interference filters transmit spectral lines of interfering gases, such as water vapor and ozone, whose concentrations were unknown and variable.
I have more fully investigated the atmospheric transmission and radiance (emission) for the two infrared bands of naturally occurring carbon dioxide in the 13 to 15 .mu.m region, and in the 3 to 5 .mu.m region, more specifically the 4 to 4.5 .mu.m region. I have found that for a temperature of about 300 K. the radiation at the 4.2 .mu.m band is about an order of magnitude less than the 14 .mu.m band, but that the 4.2 .mu.m band is about three times more sensitive to temperature changes than the 14 .mu.m band. Furthermore, in the 4.15 to about 4.2 .mu.m spectral region, one finds mainly the regularly spaced CO.sub.2 lines free of interfering spectral lines of nitrous oxide, methane, ozone, and water vapor. Indeed, within this spectral region the nitrous oxide and methane spectral lines are three orders of magnitude weaker than the CO.sub.2 lines, while the water vapor emission in this region is five orders of magnitude weaker. Thus, by tuning a radiometer to different spectral lines in the shorter wavelength CO.sub.2 band when observing the radiation obtained from an air column, data as a function of range can be obtained with the weaker spectral lines providing information at longer ranges.